----------------------------------------------------------------------- Minutes of the ABP-LCE team meeting of 26.11.04 present: EB, AG, WH, JJ, EM, TP, JPR, DS, FZ excused: FR ----------------------------------------------------------------------- (1) Pending Actions & News ---------------------------- JPR informed the team that Francesco should have returned home this week and that he is quickly recovering. He is expected to be back at CERN beginning of next year. JPR also explained that he will chair the team meeting during FR's absence to show that the section is alive and to ensure no backlog of unfinished actions will accrue. OLD ACTION -> EM will check with BT experts and/or with FC whether 4A4 ferrite data is available at higher frequencies. Otherwise some analytic model will be used to extrapolate the available data to higher frequencies. EM explained that 4A4 ferrite date were used by L. Vos and H. Tsutsui in the past, but Enrique Gaxiola informed him that all CERN kickers are made from 8C11 ferrite. Available data for 8C11 extend to 1.8 GHz only. Extrapolation to higher frequencies is unclear for 8C11. OLD ACTION -> Study sensitivity of TMCI threshold for LHC beam to various impedance parameters (EM). EM presented the dependence of the threshold on the resonator frequency (as well as on chromaticity and longitudinal emittance). A BB model for 8C11 has a higher resonant frequency than 4A4, and, hence, the threshold should shift upwards. EM also compared thresholds computed by MOSES and HEADTAIL codes with a coasting beam formula. A flat chamber plus space charge result in a markedly increased threshold compared with a round chamber and no space charge, while considering either flat chamber or space charge alone reveals little effect. FZ wondered whether the weak dependence on the geometry w/o space charge is consistent with K. Cornelis' theory. EM responded that for a shorter bunch and coupling of lower modes a stronger dependence would indeed be expected. EM pointed out that MOSES predicts a threshold of 5.5e10 protons per bunch for the SPS at injection with a longitudinal emittance of 0.2 eVs, in good agreement with observations. If space charge acts stabilizing, the real impedance of the SPS could be much larger than presently assumed. ACTION -> EM and/or FZ will review Landau-damping mechanisms. STATUS: pending. Low priority. OLD ACTION (EB) -> Plot results as a function of effective phase advance. Perform tune scan with a constant electron density Simulate the situation of the recent SPS experiments OLD ACTION -> Finer tune scan near resonance for constant density (EB) STATUS: pending. EB presented e-cloud simulations results for a density of 2e11 m^-3 (below the threshold of the fast instability). If the slow emittance growth seen for different numbers of kicks is scaled with the number of kicks and plotted vs. the fractional part of the tune divided by the number of kicks there is evidence for emittance growth near resonances, e.g. close the 1/3rd integer. In new simulations, the density was held constant and the tune was varied. These simulations even more clearly reveal a resonance fine structure. Plotting the working points (lattice tunes and peaks inferred from centroid-motion FFT) in the tune diagram did not at first glance show a clear correlation between the simulated emittance growth and nearby resonance lines. Following a suggestion made by G. Franchetti at the HHH workshop, for the first set of simulations with varying no. of kicks EB plotted the working points in the rune diagram together with only the systematic resonance lines, taking into account the 'super-periods'. The observed closest distance to a resonance line must still be weighted with the order of the resonance in an appropriate way, before comparing it with the magnitude of emittance blow up. JJ suggested a weighting scheme inferred from the KAM theorem. OLD ACTION => Clarify why B. Zotter's results are an order of magnitude larger than the results from Vos, Burov&Lebedev; BZ will give a presentation in one of the next LCE meetings. STATUS: Bruno Zotter will give a short talk mid or end November. He also recommended to move the tune closer to an integer to detect the low-frequency part of the collimator impedance in the SPS experiment. A similar suggestion was made by FZ during last week's MD. However, this could not be realized, since, approaching the integer from below - for an unstable situation - implies crossing the half integer, which according to GA is impossible at 270 GeV/c. ACTION => EM will contact B. Zotter to investigate the possibility of a presentation in the near future. (2) Intensity-Dependent Emittance Transfer (EM) ----------------------------------------------- EM followed up on the LCE meeting of 17.09.04 and ICFA-HB2004 workshop on emittance transfer when crossing the Montague resonance. A simple 2D model gives results in nearly perfect agreement with 2D simulations of the code IMPACT from LBNL. 3D simulations, with synchrotron motion, show less emittance transfer, but still much more than experimentally observed. Accordign to I. Hofmann, IBS could be responsible for this difference. EM recommended a closer look at the simple 2D model, which contains only two parameters. (3) Trip Report KEK (FZ) ------------------------ From Nov 13-8 FZ attended the 1st ILC meeting and the 6th workshop on higher luminosity B factory at KEK. He presented three invited and two contributed talks and helped preparing another invited presentation. He already gave a summary of ILC WGs 3 and 4 in a CLIC seminar. The Super KEKB upgrade aims to increase the luminosity by a factor of 10-50 up to 5e35 cm-2s-1. It involves new rf, new vacuum chambers, fingerless bellows, swapping of e+ and e- rings, a new linac section with C-band acceleration, an e+ damping ring to damp the e+ emittance after e+ production, and crab cavities. FZ also discussed CSR effects in the CLIC damping ring with T. Agoh, and an extension of the MOSES code with Y.H. Chin. (4) Reports from Meetings (JJ) ------------------------------ JJ reported that the vacuum group assures a vacuum pressure for ions which is 100 times lower than that required for protons. This level would be adequate for ion operation. JJ also summarized a recent I-LHC/LEIR meeting addressing, among other topics, software for LEIR ring, the NSAC US heavy ion program review, and TV screens for the transfer lines; the meeting minutes are found at http://carli.home.cern.ch/carli/ILHCandLEIR/Meeting_04_11_16/Summary_04_11_16.html and the review of the US heavy ion physics program at http://www.sc.doe.gov/henp/np/nsac/nsac.html (5) Preparation for Impedance Discussion at next LTC (EM et al) --------------------------------------------------------------- EM though on vacation at all Wednesdays in December agreed to give the presentation. For the LHC he presently has information on the collimators and the beam screen, which are the largest impedance sources. For the SPS we have a model of the kicker impedance and broadband- resonator model for the rest of the impedance. A complete frequency-dependent impedance model is not yet fully available. DS mentioned that the longitudinal resistive component of the collimators/kickers could matter. FZ mentioned that he has impedance information for a few other LHC items, such as rf cavities. (6) AOB ------- JPR told the team that the new group structure will be announced soon. A group drink will be organized in Room 6-2-004 on 14 December at 17:00. Attached: Slides by EB, EM (2x), FZ